shader: support DS_WRITE_B8_D16_HI (#794)

Decode GFX10 DS opcode 0xa0 as DS_WRITE_B8_D16_HI (next to
DS_WRITE_B16_D16_HI at 0xa1) and lower it through the existing 8-bit
shared-memory write. The stored byte is bits 23:16 of the data VGPR,
selected with SDWA BYTE_2 on src1.

Add captured-instruction decode coverage (from GTA V, PPSA04264) and a
runtime regression that writes one byte of a distinctive source value
into a pre-filled LDS dword.

src/graphics/shader/recompiler/frontend/decode/ShaderDecoder.h

@@ -541,6 +541,7 @@ enum class Opcode {
 	DS_READ_B128,
 	DS_WRITE_B8,
 	DS_WRITE_B16,
+	DS_WRITE_B8_D16_HI,
 	DS_WRITE_B16_D16_HI,
 	DS_WRITE2_B32,
 	DS_WRITE2ST64_B32,